1. Field of the Invention
This invention relates to the use of fatty acid derivatives as a substitute for aromatic solvent constituents in solvents for printing inks. The present invention also relates to printing inks containing binders, pigments, non-aromatic solvents and optionally additives.
2. Discussion of the Related Art
Printing products of various kinds can be produced using various printing techniques which may be divided into three main types, namely: letterpress printing, planographic printing (or even offset printing) and gravure printing. In letterpress printing, the printing ink is transferred from hard raised letters, which are covered with a thin layer of ink by rubber rollers, to the substrate. The composition of the printing ink has to be such that it dries relatively slowly and does not begin to harden prematurely. Viscous, very slow-drying printing inks are required for modern high-speed newspaper printing machines using the rotary letterpress process. In offset printing, the design to be reproduced is fixed on printing plates in the form of zones of opposite polarity. The hydrophobic, viscous printing ink only wets the hydrophobic zones on the printing plates. In gravure printing, the motif is engraved into the printing plate. After wetting of the printing plate with the relatively low-viscosity printing ink, the surface is stripped so that printing ink only remains in the engraved depressions from which it is then transferred to the substrate to be printed.
The above examples show that printing inks have to satisfy a number of requirements against the background of economy. The principal components of a printing ink are pigments, binders, solvents and additives with which the required properties of the printing inks can be varied. For example, viscosity, flow behavior and tackiness can be adjusted according to the application envisaged for the printing ink. The various requirements which the physical properties are expected to meet against the background of economy, particularly in the case of mass-produced printed products, impose stringent demands on the solvent used in the printing ink. On the one hand, it must be capable of dissolving various binders and various additives; on the other hand, it must allow viscosity to be adjusted to the required value.
By virtue of their favorable price, mineral oils have been successfully used as solvents for printing inks. However, one disadvantage of these hydrocarbon-based diluents is that their dissolving properties in regard to the binders used are poor. In the case of mineral oil for example, the binder dissolving properties deteriorate with decreasing aromatic content (Ullmann's Encyclopedia of Industrial Chemistry, A 22, 147 (1993)). In practice, therefore, solvents with--in some cases--a high content of aromatic hydrocarbons are used in printing inks to compensate for the poor dissolving properties of the aliphatic hydrocarbons in the mineral oil. In the case of offset printing for example, natural-resin-modified phenolic resins and alkyd resins modified with drying oils are dissolved in high-boiling mineral oils containing 16 to 20% of aromatic hydrocarbons (G. H. Hutchinson in "Solvent Problems in Industry", G. Kakabadse (ed.), Elsevier, N.Y. (1984)). However, the aromatic components in the solvent have an adverse effect both on its toxicology and on its environmental behavior.
The problem addressed by the present invention was to replace the aromatic constituents in solvent mixtures used for the production of printing inks by at least equally effective, but environmentally far safer substances for the reasons explained above.
It has now surprisingly been found that the aromatic constituents in solvents for printing inks can be fully replaced by fatty acid esters and fatty alcohols in various fields of application.
The use of fatty acid esters in printing inks has been known for some time. In particular, the triglycerides of unsaturated fatty acids are used for lithographic printing inks. The most commonly used oils are linseed oil, soybean oil and tall oil (Encyclopedia of Chemical Technology, 13, 381 et seq.). However, the purpose of these additives lies in their function as an oxidatively drying binder. The use of esters of unsaturated fatty acids as solvents in the production of printing inks is not apparent from the relevant works.
DE-A 43 19 825 teaches the use of a cleaning paste for removing residues and printing inks from rubber printing rollers or printing blankets. Besides abrasive components, the cleaning paste contains 10 to 25% of fatty acid esters in the form of methyl and ethyl esters. Soybean oil methyl ester and rapeseed oil methyl ester in particular are expressly mentioned. However, it is not apparent from the document in question that the fatty acid esters would also be suitable for the production of a printing ink and not only in conjunction with abrasives for cleaning inking rollers.
U.S. Pat. No. 5,340,493 is also concerned with the cleaning of printing rollers, blankets and machines. This document discloses cleaning solutions consisting of tall oil alkyl ester, organic solvent and a surfactant. The object of the invention in question was to provide a cleaning solution for parts of printing machines; there is no suggestion anywhere of its use in the production of printing inks.
In DE-C3-25 12 734, a saturated fatty acid ester is added as a release agent to an ink. The object of this is to obtain improved wiping behavior where the ink is applied to non-absorbent surfaces. However, the ink does not contain a mineral oil.
It is known from U.S. Pat. No. 3,946,138 and the associated U.S. Pat. No. 4,069,179 that printing inks used for transfer printing may contain an aliphatic monoalcohol as carrier. Cetyl alcohol, myristyl alcohol and stearyl alcohol are mentioned as examples of such an alcohol. It is apparent both from the description and from the examples that the quantity of alcohol used makes up at least 40% by weight of the ink. Mineral oils are not mentioned.